Techniques for the preparation of aqueous ink compositions are well known. For example, an aqueous ink composition can be prepared by combining the essential component, a water-soluble dye solution, with a solvent, a surface active agent, a pH adjustor, and a preservative.
In recent years, ecological and safety problems related to coloring materials and their components have received attention. However, the attention has been primarily focused on oily inks, which include harmful volatile components. The relatively minor effects of aqueous ink compositions have been largely ignored. As a result, materials for use in aqueous ink compositions are often selected with performance and cost, not safety, as the overriding considerations.
This view is shortsighted, however, because harmful bacteria can contaminate and even thrive in aqueous ink compositions. Such contamination can not only adversely influence the performance of the ink, i.e., deteriorate writing performance and generate a unpleasant smell, but it can also have a detrimental effect the user, who can unconsciously inhale bacteria or mold spores.
This problem is conventionally addressed in aqueous ink compositions by the addition of a preservative which retards the growth of harmful bacteria and mold. The preservatives are generally effective by inhibiting the development or by killing the harmful microorganisms. However, the more effective preservatives tend to be toxic not only to the bacteria and mold, but also to humans, thus creating a safety concern for their use.
Japanese Patent Application Laid-open Nos. (Hei) 4-298577 and (Hei) 4-318081 disclose an aqueous ink composition that includes a dye as an essential component and additional necessary components such as a solvent, a surface active agent, a preservative and a pH adjustor. All components are described as food additives or conformed materials. Furthermore, both publications suggest the use of pH adjustors in aqueous ink compositions, such as citric acid, tactic acid, malic acid, succinic acid, and gluconic acid, which are generally described as sour seasonings. Further, the documents teach the use of phosphoric acid and ammonium sulfate which are generally used as brewing materials, and, further, sodium carbonate, sodium hydrogen carbonate, potassium carbonate and dipotassium phosphate, which are alkali materials for food manufacture, as described in the Japanese Standards for Food Additives (Tokyo, Ministry of Health and Welfare, 6th ed).
However, the described pH adjusters of the aqueous ink composition fail to maintain the pH of the composition over time. This results in alteration of ink color and breakdown of the ability of the preservative to avoid microbial contamination. Therefore, there is a need in the art for safe aqueous ink compositions which maintain the desired pH value over time.